Power cable 1480601 225916461 2008-07-16T00:22:35Z GargoyleBot 6678853 Remove per [[Wikipedia:Templates_for_deletion/Log/2008_July_4#Template:Redirectstohere|TfD]]. {{two other uses|electric power conveyances|the fictional town|Power Cable|portable equipment |Power cord}} [[Image:ElectricWireGrounded.jpg|thumb|right|150px|A electrical power cable found commonly in modern European houses. The cable consists of 3 wires (2 wires + 1 grounding) and is double-insulated]] A '''power cable''' is an [[assembly]] of two or more [[electrical]] [[conductor (material)|conductors]], usually held together with an overall [[sheath]]. The assembly is used for transmission of [[electric power|electrical power]]. Power [[cable]]s may be installed as permanent [[wiring]] within buildings, buried in the ground, run overhead, or exposed. Flexible power cables are used for portable devices, mobile tools and machinery. ==History== Early [[telegraph]] systems used the first forms of electrical cabling, transmitting small amounts of power. [[Gutta-percha]] insulation used on the first [[transatlantic cable]]s was unsuitable for building wiring use since it deteriorated rapidly when exposed to air. The first power distribution system developed by [[Thomas Edison]] used [[copper]] rods, wrapped in jute and placed in rigid pipes filled with a bituminous compound. Although vulcanized rubber had been patented by Charles Goodyear in 1844, it was not applied to cable insulation until the 1880s, when it was used for lighting circuits.<ref>''Underground Systems Reference Book'', Edison Electric Institute, New York, 1957, no ISBN </ref> Rubber-insulated cable was used for 11,000 volt circuits in 1897 installed for the [[Niagara Falls]] power project. Oil-impregnated paper-insulated high voltage cables were commercially practical by 1895. During [[World War II]] several varieties of synthetic rubber and [[polyethylene]] insulation were applied to cables.<ref> R. M. Black The History of Electric Wires and Cables, Peter Pergrinus, London 1983 ISBN 0 86341 001 4 </ref> ==Safety== Because these cables normally carry high voltages (nominally 110 volts in North America, 220 volts in Europe, with variations in the rest of the world) they are dangerous if misused in service, or if damaged accidentally, and are often lethal. This occurs for two reasons. Direct electrocution of humans or animals through which accidental current might flow in case of a failure, or because there is enough current available that imperfect connections (for instance, a loose screw around which terminating wire is wrapped at a connection), or inadvertent contact with machinery or structures, can generate enough heat to start a fire which may in turn kill and often causes considerable damage and loss. This is the reason why construction techniques generally include safety grounds, and why sheathing materials are specified for certain conditions and prohibited from use in others. In the first case, fires and lethal current exposure is reduced by providing a way to force circuit protectors (fuses or circuit breakers) to trip. In the second case, particular materials resist deterioration from certain causes, also reducing exposure risks. The matter is sufficiently important that fire and building regulations in many places require certain types of power cable in certain uses. ==Construction== Modern power cables come in a variety of sizes, materials, and types, each particularly adapted to its uses.<ref> Terrell Croft and Wilford Summers (ed), American Electricans' Handbook, Eleventh Edition, McGraw Hill, New York (1987) ISBN 0-07013932-6, sections 2-13 through 2-84 </ref> Large single insulated conductors are also sometimes called power cables in the industry.<ref> Donald G. Fink and H. Wayne Beaty, Standard Handbook for Electrical Engineers, Eleventh Edition,McGraw-Hill, New York, 1978, ISBN 0-07020974-X pg. 18-85 </ref> Cables consist of three major components: conductors, insulation, protection. The makeup of individual cables varies according to application. The construction and material are determined by three main factors: * Working voltage, determining the thickness and composition of the insulation; * Current-carrying capacity, determining the cross-sectional size of the conductors; * Environmental conditions such as temperature, chemical or sunlight exposure, and mechanical impact, determining the form and composition of the outer cable jacket. Power cables use stranded copper or aluminum conductors, although very small power cables may use solid conductors. The cable may include uninsulated conductors used for the circuit neutral or for ground (earth) connection. The overall assembly may be round or flat. Non-conducting filler strands may be added to the assembly to maintain its shape. Special purpose power cables for overhead or vertical use may have additional elements such as steel or [[Kevlar]] structural supports. For circuits operating at or above 2,400 volts between conductors, a conductive shield may surround each conductor. This equalizes electrical stress on the cable insulation. This technique was patented by [[Martin Hochstadter]] in 1916;<ref> Underground Systems </ref> the shield is sometimes called a Hochstadter shield. The individual conductor shields of a cable are connected to earth ground at one or both ends of each length of cable. Some power cables for outdoor overhead use may have no overall sheath. Other cables may have a plastic or metal sheath enclosing all the conductors. The materials for the sheath will be selected for resistance to water, oil, sunlight, underground conditions, chemical vapors, impact, or high temperatures. In nuclear industry applications the cable may have special requirements for ionizing radiation resistance. Cable materials may be specificed not to produce large amounts of smoke if burned. Cables intended for underground use or direct burial in earth will have heavy plastic or [[lead]] sheaths, or may require special [[direct-buried cable|direct-buried]] construction. When cables must run where exposed to mechanical impact damage, they are protected with flexible steel tape or wire armor, which may also be covered by a water resistant jacket. Cables for high-voltage (more than 65,000 volts) power distribution may be insulated with oil and paper, and are run in a rigid steel pipe, semi-rigid aluminum or lead jacket or sheath. The oil is kept under pressure to prevent formation of voids that would allow [[partial discharge]]s within the cable insulation. Newer high-voltage cables use cross-linked polyethylene (XLPE) for insulation. A hybrid cable can include conductors for control signals or may also include [[fiber optics|optical fibers]] for data. == Named cable types == Common types of general-purpose cables used by [[electrician]]s are regulated by national or international codes, which define the various wire alloys that may make up a cable, its insulation type and characteristics, including its resistance to chemicals and sunlight. Commonly-used types of power cables are often known by a "shorthand" name, and today many contain an uninsulated bare wire which is for connections to [[earth ground]] and is for safe-guarding local junction and plug boxes for consumer and fire safety. In the latter case, in theory, a fireman's axe cutting through a ground-equipped cable will provide a short circuit and blow the fuse or breaker, so the whole circuit becomes dead, and shock risks are minimized. Three prong power outlets and plug-cords require a ground wire. Extension cables often have an insulated ground wire. Official names and shorthand names often follow the [[National Electrical Code (US)|USA's NEC]] type ''NM-B'' (''Non-Metallic, variant B'', which coding refers to the insulation properties coating each working wire, the sheath, and the heat-, chemical- and sunlight-resistant characteristics of the plastic insulators), often referred to as "Romex"<sup>TM</sup>,<ref>"Romex"<sup>TM</sup> - named by the [[Rome Wire Company]], now a trademark of Southwire Company [http://www.southwire.com]</ref> is a cable made of solid copper wires with a nonmetallic plastic jacket containing a waxed paper wrapped inner group of at least a pair of plastic insulated service wires and a bare ground wire that is characteristic of modern electrical cables. The most common Romex/NM-B cable thus has three wires: the return wire (called "neutral", usually colored white), the wire providing power to the load, which is nearly always black, and the naked ground wire. NM-B "two wire" cables are available in different wire gauges, providing a range of current-carrying capacities at expected maximal ambient temperatures in a cable run. {|style="float:right;clear:left; padding: 1em; margin: 1em 0 1em 2em; text-align:center; font-size:75%; border: 1px solid #a9a9a9; background:#a9f9c9; width:50%;" |- !Romex<sup>TM</sup> wire<br/>label type || count of <br/>insulated wires ||Colors<br/>(normal) || total <br/>wires || wire gauge<br/>rating |- | 12-2 NMB || 2|| WH + BK || three || AWG 12 |- | 14-2 NMB || 2|| WH + BK || three || AWG 14 |- | 12-3 NMB || 3|| WH + BK + RD || four ||AWG 12 |- | 14-3 NMB || 3|| WH + BK + RD || four ||AWG 14 |- |colspan=5 |"AWG" means [[American Wire Gauge]], see table [[American_Wire_Gauge#Table_of_AWG_wire_sizes|here]] for current capacities derated for various maximum expected temperatures within a cable run or [[conduit]]. Other commonly used wire gauges for common electric services distribution range from AWG-4 through AWG-10. |} The next most common "Romex" variant (sometimes called a "Three way" cable, after its application in lighting) has the return (white/neutral) and two power sourcing wires: the first conductor is always black and the second conductor is usually colored red (sometimes yellow or blue, ''but only rarely''&mdash;such colors must be specially ordered and are more expensive) and an uninsulated copper ground wire for fire safety. This type is generally used for multiple switching locations of a common or shared lighting need, such as for switches located at either end of a long commercial office suite, hallway, or on both upper and lower floors for stairway lighting, and are called a "three-way circuit": a well-entrenched [[misnomer]] in North America. Similarly, three or more switches directly controlling a light require one or more "[[Switch#More than two locations|four-way switches]]" plus a pair of "[[Switch#Two locations|three-way switches]]"). Type NM-B cables with three insulated conductors are also suitable for larger loads like clothes dryers, heavy duty air conditioners and electric heaters that require both hot legs for twice the normal voltage. Thus for NEC NM-B series/Romex<sup>TM</sup> cables, at least two insulated wires plus the bare copper ground wire are always present. In general, NEC NM-B/Romex wire is not suitable for enclosure with other wiring in conduits, especially outdoor conduits susceptible to solar heating, which requires a higher temperature plastic insulation such as "THHN" (stranded, sold in rolls of single conductor in many colors). Another common nickname is type ''UF'' (''underground feeder'') which has a nonmetallic protective sheath but uses a moisture- and sunlight-resistant construction suitable for direct burial in the earth, where exposed to sunlight, or in unusually wet, unusually dry, or corrosive locations. "UF" uses a sheath that is a flexible solid about the insulated wires instead of the waxed paper wrapping, is generally heavier, less flexible than NM-B, and is rated against breakdown from harmful [[ultraviolet]] light wavelengths commonly encountered in direct sunlight. Another common type is "AC", a fabricated assembly of insulated conductors in flexible metallic armor, formed by wrapping an interlocking metal strip around the conductors. This is a descendant of type "''BX''", an early [[genericized trademark]] of the [[General Electric]] company, used from before and during World War II, designating a particular design of armored cable. The "BX" name is commonly applied loosely to all flexible metal armored cables including styles with individual wires running through a flexible metal conduit of similar appearance. In Canada, type TECK cable, with a flexible aluminum or steel armor and overall flame-retardant [[polyvinyl chloride|PVC]] jacket, is used in industry for wet or dry locations, run in trays or attached to building structure, above grade or buried in earth. A similar type of cable is designated type "MC" in the United States. Electrical power cables are often installed in raceways, including [[electrical conduit]] and cable trays, which may contain one or more conductors. [[Mineral-insulated copper-clad cable]] (type ''MI'') is a fire-resistant cable using [[magnesium oxide]] as an insulator. It is used in demanding applications requiring superior heat resistance such as [[fire alarm]]s and [[oil refinery|oil refineries]]. ==Flexible cables== All electrical cables are somewhat flexible, allowing them to be shipped to installation sites wound on reels or drums. Where applications require a cable to be moved repeatedly, more flexible cables are used. Small cables are called "cords" (North American usage) or "flex" (United Kingdom){{Fact|date=March 2008}}. Flexible cords contain finer stranded conductors, not solid core conductors, and have insulation and sheaths that are engineered to withstand the forces of repeated flexing and abrasion. Heavy duty flexible power cords such as those feeding a mine face cutting machine are carefully engineered—their life is measured in weeks. Very flexible power cables are used in automated machinery, [[robotics]], and machine tools. See "[[power cord]]" and "[[extension cable]]" for further description of flexible power cables. Other types of flexible cable include [[twisted pair]], extensible, [[coaxial cable|coaxial]], [[shielded cable|shielded]], and [[communication cable]]. == See also == * [[Electrical wiring]] * [[American wire gauge]] -- For a table of cross section sizes * [[Ampacity]] -- For a description of current carrying capacity of wires and cables * [[Voltage drop]] -- For another consideration necessary in selecting proper cable sizes * [[Cross-linked polyethylene]] * [[Ethylene propylene rubber]] (EPR) * [[Restriction of Hazardous Substances Directive]] * [[Portable cord]] ==References== <references/> ==External links== {{Commonscat|Power cables}} * [http://wiki.diyfaq.org.uk/index.php?title=Cables British Cables] [[Category:Power cables|*Power cable]] [[Category:Electrical wiring]] [[da:Elektrisk kabel]] [[fa:کابل برق]] [[no:Elektrisk ledning]] [[pl:Kabel elektryczny]] [[ro:Cablu electric]] [[uk:Кабель]] [[zh:電纜]]